A protest of a four-state oil pipeline turned violent after tribal officials say construction crews destroyed American Indian burial and cultural sites on private land in southern North Dakota.

Morton County Sheriff's Office spokeswoman Donnell Preskey says four security guards and two guard dogs were injured after several hundred protesters confronted construction crews Saturday afternoon at the site just outside the Standing Rock Sioux reservation.

Tribe spokesman Steve Sitting Bear says protesters reported that six people had been bitten by security dogs, and at least 30 people were pepper-sprayed.

Preskey says there were no law enforcement personnel at the site when the incident occurred. She says the crowd disbursed when officers arrived and no one was arrested.